It is well established that pipelines employed in connection with low temperature service, e.g., in the transport of products such as butane, ammonia, propane, ethane, ethylene, methane, etc., in cold liquid form, generally require suitable devices to absorb changes in line length as a result of temperatures introduced by such service. Typically, such pipelines are installed at ambient temperatures. Products like those above are typically transported in cold liquid form at much lower temperatures, e.g., at or near +30.degree. F. for butane, -28.degree. F. for ammonia, -44.degree. F. for propane, -128.degree. F. for ethane, -155.degree. F. for ethylene, -259.degree. F. for methane, etc. The different temperatures, i.e. ambient versus cold-liquid, commonly bring about significant changes in line length on pipeline commissioning as a result of pipeline thermal contraction. During normal operations such lines generally undergo lesser thermal movements in response to relatively small temperature changes related to fluctuations in ambient conditions, product compositions, or pressures.
Above ground, it is commonplace to employ in such lines loops and bellows arrangements to absorb the relatively large initial contraction movements that attend commissioning and the generally smaller movements that accompany normal operations. Each of these types of devices provides for such pipeline movements by compensatory movements by the devices themselves. To provide for this capability in loops normally demands relatively large amounts of unrestricted space inside which the loops can function. To provide for this capability in bellows normally demands that the bellows possess relatively thin walls and also a measure of unrestricted space for functioning.
Because of these requirements neither loops nor bellows are completely satisfactory for pipelines for low temperature service which must be buried underground or submerged in water. In these environments without surrounding protection, earth, sediments, growths, etc., can cause such devices to malfunction. When loops or bellows are rendered incapable of providing their intended compensatory movements, stress levels in the pipelines involved are affected and can rise to unacceptable levels. Theoretically, protective enclosures, e.g., vaults, can be installed around loops inground or underwater to prevent their being fouled. However, loops inherently lack compactness; such enclosures tend to be large and uneconomic. Bellows are by comparison compact and relatively easy to protect from external fouling; however, the characteristically thin walls of bellows are vulnerable to excesses of internal pressure and to temperature-related cyclically induced fatigue.
At present alternatives to the use of such devices involve restraint and prestress systems. In concepts involving restraint, pipelines susceptible to longitudinal shrinkage, or shortening, as a result of temperature drops, are literally restrained from shrinking or shortening. Such restraint of shrinkage causes longitudinal pipeline stress. With full restraint and too large a temperature drop such stress can cause pipeline ruptures. In prestress systems a reverse pattern of longitudinal stress must be locked into lines prior to commissioning. Generally, special restraint systems or anchorages and special prestressing devices are necessary to accomplish this end.
Pipelines for low temperature service commonly require insulation to retard influx of heat and consequent boil-off of product. Water impermeable cover over the insulation must be provided generally when such lines are buried underground (because of the usual presence of moisture) and especially when such lines are submerged directly in water (generally referred to as submarine lines), since present forms of insulation lose resistance to heat flow on infiltration by water. The deployment of such insulation and protective cover is made complex around bellows and loops by their configurations and required patterns of movement. The primary purpose of such insulation is compromised to varying extents in most restraint and prestress systems since, in most, the restraint or anchorage mechanisms extend to the cold liquid carrier and thereby provide direct conductance paths for flow of heat to the carrier and hence to the liquid.
With consideration for the above, the subject invention is directed to mechanisms which are stronger than bellows, more compact than loops, generally adaptable to fitting within a smooth outside impermeable cover insulation, e.g., a continuous carbon steel outer casing, with minimal presence of intervening heat conductors (between carrier and casing).
In connection with the subject invention, prior art which can be referred to for detailed discussions and analysis of the problems associated with cold temperature service pipelines, especially for undersea use, can be found in an article entitled "Prestress Piping System Favored for LNG Transport", The Oil and Gas Journal, pages 179 through 182, Apr. 1, 1968, by M. B. Gardner, Jr., and also in Gardner's U.S. Pat. No. 3,530,680. Other prior patents include: U.S. Pat. No. 3,379,027; U.S. Pat. No. 3,388,724; U.S. Pat. No. 3,547,161; U.S. Pat. No. 3,693,665 and Canadian Pat. No. 861,306.